English

Recovering Kodaira types from $\ell$-torsion on elliptic curves

Number Theory 2026-07-02 v1

Abstract

The classical N\'eron-Ogg-Shafarevich criterion characterises good reduction of an elliptic curve EE over a pp-adic field via the action of inertia on the \ell-adic Tate module. However, the action of inertia on E[]E[\ell] is not sufficient to distinguish between potentially good and multiplicative reduction, and the action on T(E)T_{\ell}(E) is not sufficient to determine the Kodaira type. We remedy this situation by endowing E[]E[\ell] with a distance function that records the pp-adic distances between the xx-coordinates of the points. We show that, equipped with this additional structure, E[]E[\ell] determines the Kodaira type of the elliptic curve. In the case of residue characteristic 22, we assume that EE does not have potentially good reduction of type InI_n^*.

Keywords

Cite

@article{arxiv.2607.02678,
  title  = {Recovering Kodaira types from $\ell$-torsion on elliptic curves},
  author = {Naina Praveen},
  journal= {arXiv preprint arXiv:2607.02678},
  year   = {2026}
}

Comments

11 pages. First version. Comments welcome!